Latent image structure

ABSTRACT

A latent image structure of a highly secure nature in which two latent image are superimposed. Each of the superimposed latent images is viewable from a different line of sight. Relief elements define, for each latent image, portions of respective linear relief structures providing an image and a background which cooperate to generate the latent image. Relief elements are only provided at locations where the linear relief structures of the first and second latent images intersect. The resultant latent image structure, when viewed normally, has the advantage of a relatively flat appearance, making it difficult to discern the presence either of the superimposed latent images.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a latent image structure, for example for useas a security device.

2. Description of the Related Art

A latent image is an image which cannot be seen when viewed normally(i.e. perpendicularly) but can be seen when the surface carrying theimage is held at an acute angle to the eye. Conventional latent imagesare formed as relief structures which have sufficient height to maskcertain regions when the structure is held at an acute angle. Theserelief structures can be form(ed by embossing or printing, typicallyintaglio printing. A number of different types of intaglio printedlatent images are described in U.S. Pat. No. 4,033,059. In addition,this document discloses complex, multiple latent images which enablemore than one latent image to be viewed when the structure is viewed atdifferent angles. This is particularly useful where the structure is tobe used as a security device.

For example, in U.S. Pat. No. 4,033,059, two superimposed latent imagesare described, each being formed by continuous lines. Where the twostructures overlap, a different line structure is used which means thatwhen the features are viewed from a shallow angle, the strength of theimage will vary according to whether or not the image forms part of thesecond image area. In another form, a set of dots is used. A regular dotstructure forms a bulk of the feature area with strategically placeddots so produce images in the required direction resulting in acombination of latent and transient images. This means that the presenceof an image is easily seen at normal viewing and furthermore a shadowlatent image only can be produced.

A problem which can arise with latent images is that although they areprimarily visible only when viewed at an acute angle, nevertheless theyrequire different line structures which are at least partially visiblewhen the device is viewed normally. This is true, for example, of thestructures described in U.S. Pat. No. 4,033,059 and reduces the securitynature of the device. This problem becomes even more noticeable withmultiple latent images.

SUMMARY OF THE INVENTION

In accordance with the present invention, a latent image structurecomprises an array of relief elements defining

i) a first latent image viewable along a first line of sight offset tothe normal;

ii) a second latent image viewable along a second line of sight offsetto the normal, the second line of sight being rotated laterally relativeto the first line of sight about a normal to the structure, the firstand second latent images being superimposed,

wherein the relief elements define, for each latent image, portions ofrespective linear relief structures providing an image and a backgroundwhich cooperate to generate the latent image, and wherein the reliefelements are only provided at locations where the linear reliefstructures of the first and second latent images intersect.

We have devised a new latent image structure which has the significantadvantage that when viewed normally it presents a relatively flatappearance from which it is difficult to discern the presence of the oreach latent image. Typically, the relief elements are arranged in setsof regular arrays which are identical in form but differing inorientation in contrast to previous structures. For example, the dotrelief structures described in U.S. Pat. No. 4,033,059 are irregularwith the result that the latent image can be discerned when viewednormally.

A further advantage of arranging the relief elements in a set of regulararrays is that the amount of ink used per unit area, in the case wherethe relief elements are printed, will be substantially the same acrossthe feature and this leads further to the flat appearance of thestructure and increases the difficulty of discerning the latent imagewhen viewed normally.

Typically, each linear relief structure is based on a pair of arrays oflines, the lines of each array being substantially parallel, and thelines of one array extending at substantially 90° to the lines of theother array. Of course, the relief elements themselves in this case areonly based on lines, the lines themselves not existing. Instead, therelief elements will be in the form of discrete features such aslozenges, ellipses or dots.

The relief elements may be formed by raised portions of a substrate, forexample by embossing, or by portions of ink, for example by intaglioprinting.

Typically, the lines on which the relief structure is based have asubstantially constant width although in some cases, the lines of atleast one of the linear relief structures vary in width along theirlength. This may be a pseudo-random variation. This approach isdescribed in more detail in WO-A-98/47715.

Where the structure is printed, typically all relief elements will bedifferent colours from the underlying substrate on which the reliefelements are provided. The underlying substrate could be coloured, whiteor patterned. The relief elements preferably have a dark colour such asblack, blue, purple or green or may be of more than one colour. In somecases, the substrate could comprise self-supporting carrier such as apaper or plastics layer while in other cases the substrate may comprisea carrier on which is provided a coating, for example a varnish, lacqueretc. or printed ink or a foil. This allows a wide variety of effects tobe achieved.

In some examples, each line of the linear relief structure has a widthin the range 1-250 microns.

If the relief structures are printed, for example intaglio printed, thenpreferably the widths of the relief structures lie in the range 130-440microns, preferably 170-250 microns.

Where the relief structures are blind embossed, thinner widths can beachieved, for example in the range 1-40 microns, preferably 1-10microns.

Typically, the spacing between adjacent lines of each linear reliefstructure is a similar order of magnitude to the line width. Forexample, for printed relief elements, a spacing of 170-260 microns ispreferred.

The latent image structure may be used in a variety of applications, butis particularly suitable as a security feature. Typical examples ofitems which are secured using latent images include passports,passbooks, tickets, permits, licences, financial transaction cardsincluding cheque guarantee cards, charge cards, credit cards, cashwithdrawal cards, electronic funds transfer cards, service entitlementcards, personal or article identification cards, prepayment cards,telephone cards, variable e.g. decrementing value cards, bonds, fiscaldocuments, bank notes, cheques including travellers cheques, vouchers,brand identification labels, tamper resisting or indicating labels.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a latent image structure according to the invention willnow be described with reference to the accompanying drawings, in which:

FIG. 1 is an enlarged view of a first latent image structure;

FIG. 2 is an enlarged view of a second latent image structure; and,

FIG. 3 is an enlarged view of the resultant latent image structureproduced by combining the structures of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

A first latent image structure is shown in FIG. 1 and comprises a set oflines 1 extending diagonally from top left to bottom right defining abackground, and a set of lines 2 extending at 90° to the lines 1defining a letter “D”. The lines 1,2 will have the same colour and haveheights, widths and spacing such that when viewed normally the letter“D” is not easily discernable but becomes clearly visible when viewed atan acute angle, in this case 45°.

FIG. 2 illustrates a second latent image structure defined by a firstarray of lines 3 extending from left to right and defining a backgroundand a second array of lines 4 extending vertically and defining thenumeral “20”.

Each of the structures shown in FIGS. 1 and 2 is conventional in its ownright and is based, when intaglio printed, on the disclosures in U.S.Pat. No. 4,033,059. Typical line widths range from 130 μm to 250 μm,with line repeats of 220 μm to 440 μm and line to space ratios of30-70%. It should be noted, however, that the line structures are notnecessarily the same for the first and second images although in thisexample they are shown to be substantially the same.

When the two structures are superimposed, a new structure shown in FIG.3 is produced formed by a regular array of similarly shaped reliefelements 5, each relief element being provided at the intersectionbetween superimposed lines of the two latent image structures of FIGS. 1and 2. In addition, any areas corresponding to spaces in either of thetwo latent images and their backgrounds will be spaces in the finishedfeatured.

It will be immediately apparent from FIG. 3 that, even at this enlargedscale, the latent images are much more difficult to discern when viewednormally. This arises from a number of factors including the fact thatthe vast majority of the relief elements have substantially the same (inthis case lozenge) shape, and the amount of ink used per unit area issubstantially the same.

A further advantage of this arrangement is that both latent images willhave substantially the same strength.

In the preferred arrangements, the relief elements will be printed,typically intaglio printed, on a substrate but in some cases they couldbe defined by raised portions of the substrate itself, for example byembossing. Where the relief elements are intaglio printed, one or moredifferent colour splits may be introduced within or across the printedfeature.

Furthermore, with the present invention, each latent image can be seenin shadow and by turning the paper through 90° in highlight in contrastto those of U.S. Pat. No. 4,033,059.

What is claimed is:
 1. A latent image structure comprising an array ofrelief elements defining i) a first latent image viewable along a firstline of sight offset to the normal; ii) a second latent image viewablealong a second line of sight offset to the normal, the second line ofsight being rotated laterally relative to the first line of sight abouta normal to the structure, the first and second latent images beingintegrated, wherein the relief elements define, for each latent image,portions of respective linear relief structures providing an image and abackground which cooperate to generate the latent image, and wherein therelief elements are only provided at locations where the linear reliefstructures of the first and second latent images intersect.
 2. Astructure according to claim 1, wherein each linear relief structure isbased on a pair of arrays of lines, the lines of each array beingsubstantially parallel, and the lines of one array extending atsubstantially 90° to the lines of the other array.
 3. A structureaccording to claim 2, wherein the lines defining the first latent imageare at substantially 45° to the lines defining the second latent image.4. A structure according to claim 1, wherein the relief elements are inthe form of lozenges, ellipses or dots.
 5. A structure according toclaim 1, wherein the relief elements are defined by raised portions of asubstrate.
 6. A structure according to claim 5, wherein the reliefelements are embossed in the substrate.
 7. A structure according toclaim 1, wherein the relief elements comprise portions of ink.
 8. Astructure according to claim 7, wherein the relief elements are intaglioprinted on a substrate.
 9. A structure according to any of the precedingclaims, wherein each line of the linear relief structure has a width inthe range 1-250 microns.
 10. A structure according to claim 7 or claim8, wherein each line of the linear relief structures has a width in therange 130-440 microns, preferably 170-250 microns.
 11. A structureaccording to claim 10, wherein the centre-to-centre spacing of adjacentlines of each linear relief structure lies in the range 170-250 microns.12. A structure according to any of claims 1 to 8, wherein each line ofthe linear relief structures has a width in the range 1-40 μm,preferably 1-10 μm.
 13. A structure according to any of the precedingclaims, wherein the line to space ratio of each linear relief structureis in the range 30-70%.
 14. A structure according to any of thepreceding claims, wherein the lines of at least one of the linear reliefstructures vary in width along their length.
 15. A structure accordingto claim 14, wherein the variation is a pseudo-random variation.
 16. Asecurity document carrying a latent image structure according to any ofthe preceding claims.